The present invention relates to an optical module that is useful for a communication system using an optical fiber. To be more specific, the present invention relates to an optical module that has an optical fiber, or an optical connector, for inputting or outputting a light signal, and a terminal for inputting or outputting an electric signal, and that has a light-emitting device or a detector inside the optical module. In particular, a package main body is formed of a ceramic plate multi-layer structure, and the present invention is useful for an optical nodule which is intended for ultrahigh-speed operation at 10 Gbit/s or more.
In recent years, the speed of optical communication systems is rapidly becoming faster, and prices thereof are also rapidly becoming lower. As a result, a faster optical module, a price of which is lower, is strongly demanded. As a package of the optical module, a plastic package, a metal package, a ceramic package, and the like, are being examined. However, an optical module intended for high-speed operation at 10 Gbit/s or more needs the connection of input and output terminals (leads) and a light-emitting element or a detector by incorporating a microwave line, a strip line, a microstrip line, a coplanar line, or the like, each impedance of which is matched, in this optical module in order to transfer an electric signal with low loss and low distortion. It is also necessary to connect a terminal or lead for inputting and outputting to a luminous element or a light-receiving element. Therefore, a high-speed optical module inevitably uses a ceramic package capable of including a built-in microwave line. The ceramic package is formed of a multilayer structure of ceramic, and is produced by a method called a green-sheet method. Using this method, an electrode pattern or a via hole can be formed arbitrarily on each layer, and thereby a high-frequency circuit, impedance of which is matched, can be provided on a package.
Usually, hermetic sealing of an optical module using a ceramic package can be achieved by using a metal cap. In addition, the metal cap is electrically connected to a terminal (pin) which is connected to a ground so that the cap functions as an electromagnetic shield. This reduces degradation in performance of the module caused by circumferential electromagnetic noise, and also reduces electromagnetic noise emitted from the module in reverse. In other words, the metal cap improves an EMC (electromagnetic compatibility) property of the optical module. In the optical module using the ceramic package, the first conventional method for securing a metal cap is welding; for example, the method is described in Japanese Patent Laid-open No. Hei 7-63957, Japanese Patent Laid-open No. 2000-164742, and the like. The welding ensures hermetic, and what is more, the welding can achieve excellent continuity with an electrode pattern provided on the package side. Connecting the electrode pattern to a ground pin permits the cap to function as an electromagnetic shield. The second conventional method is solder jointing: for example, the method is described in Japanese Patent Laid-open Nos. Hei 10-293230, Hei 10-170771, Hei 9-318849, and the like. The solder jointing can also ensure hermetic and continuity. Moreover, the third method is described in Japanese Patent Laid-open No. Hei 10-12808. In this method, a metal shield cap is secured to a multilayer ceramic substrate at low cost. This method relates to a RF power amplifier module used for mobile devices. According to the method, the metal cap is temporarily secured to the multilayer ceramic substrate by means of a mechanical structure; and when mounting the module to a mounting substrate by soldering, fusing solder and joining the cap to the substrate with the solder achieves continuity between the shield cap and a ground.
An object of the present invention is to provide an optical module which uses a ceramic package, and is characterized by the following: low cost; an optical part is not damaged; and the metal cap can be secured while conductivity is reliably provided. According to the present invention, it is possible to provide, at low cost, an optical module that operates at super-high speed of 10 Gbit/s class, and that has an excellent EMC property.
As a background of the present invention, the conventional methods have the following disadvantages:
If a cap is secured by the first welding method, it is necessary to provide, in advance, voluminous metal (a ring for welding), which melts by welding, on the ceramic package side. More specifically, in advance, by means of Ag brazing solder, or the like, a metal ring such as kovar should be secured to the electrode pattern which is provided on the surface of the ceramic package. Therefore, the cost of the welding method is extremely high, which is a disadvantage to be considered.
In addition, if the metal cap is secured by solder jointing which is the second method, heating at about 200xc2x0 C. is required. This produces a problem of thermal resistance of an optical part and an adhesive. For example, in an optical module having a pigtail of an optical fiber, heating changes the quality and shape of a sheath (nylon, etc.) of the fiber. In the worst case, an imposed stress causes microbending in a core wire of the fiber, resulting in a large optical loss. An optical module on which an optical connector for inputting and outputting a light signal is mounted also uses an adhesive made of epoxy, etc. to secure a ferrule constituting a connector to a module. Therefore, it has the following disadvantages: if the optical module is heated at about 200xc2x0 C., the adhesive degrades, leading to a decrease in bond strength; and fitting the connector into the optical module, and taking the connector out from the module, increase an optical loss of the connector.
Furthermore, the third method also has a disadvantage that implementing an optical module on a mounting substrate by a reflow device causes heat damage to optical components.
Basic thoughts of the present invention will be described below.
The present invention provides an optical module comprising at least: a module base; a lid member having conductivity for covering the module base, the module base and the lid member forming a space therebetween; at least a semiconductor optical element and an electric signal wiring portion disposed in the space; and an optical path member which leads from the space to the outside of the module base, wherein the module base is made of ceramic; at least a part of the module base has a conductive member; at least a part of the conductive member has a conductive adhesive; the module base and the lid member are bonded together by the conductive adhesive; a conductive function portion possessed by the lid member is electrically connected to the conductive member which is provided on at least a part of the module base; the conductive member is electrically connected to a terminal which is connected to a ground; and the conductive adhesive is an organic conductivity adhesive.
Referring to typical and more specific modes, the present invention will be described in detail hereinafter.
One mode is an optical module comprising: an optical fiber or an optical connector, which is used for inputting and outputting a light signal; a terminal for inputting and outputting an electric signal; a main body, namely, a module base; a lid member, namely, a cap; a semiconductor optical element or a semiconductor integrated circuit element in a space surrounded by the main body and the cap, wherein ceramic plates, each having a wiring pattern, are laminated to form the multilayerd main body. The main body is provided with a microstrip line for transmitting a high-speed electric signal, and a high-frequency circuit including a via hole, a terminating resistor, a wire bond, and the like; and the terminal is connected to the semiconductor optical element or the semiconductor integrated circuit element. An electrode pattern is provided on at least a part of a surface of the main body; and the electrode pattern is connected to the terminal which is connected to a ground. A conductive adhesive is applied to at least a part of the electrode pattern to bond the cap.
As the cap, a metal cap, or a cap having a conductive film on its surface, is used. As a base material of the cap, besides metal, for example, ceramic may also be used, and a conductive film may be formed on its surface. Electrically connecting the lid having conductivity to the terminal which is connected to a ground permits the lid to function as an electromagnetic shield. Thus, degradation in property of the module caused by circumferential electromagnetic noise can be prevented, and emission of electromagnetic noise from the module can also be prevented.
The conductive adhesive is an organic adhesive containing conductive filler and also containing thermosetting resin or thermoplastic resin; for example, it is a silver epoxy adhesive in which epoxy resin is mixed with a hardening agent and silver dust. A conductive adhesive is selected so that curing temperature of the conductive adhesive becomes at least 150xc2x0 C. or less, or 130xc2x0 C. or less if possible. In general, such a conductive adhesive is generally called a cold cure conductive adhesive.
The semiconductor optical element inside the package is protected with transparent resin. As protective coating, general resin used for protective coating suffices. A typical example of such a resin is silicon resin gel. The package is provided with a ventilation structure so that water in the package can go in and out easily. Examples of such a ventilation structure will be described later.
As a material of the metal cap, a material having an expansion coefficient which is close to that of the ceramic package is preferable; for example, kovar, FeNi alloy, and stainless steel are preferable. Plating may be applied to the surface of the metal cap; for example, plating including Au and Pd is performed. In addition, if a conductive film is used, ceramic, resin, or the like, is used for the main body of the cap. The cap has a bent portion to cover at least a part of a pair of sides of the main body. Fittable areas are formed at, at least the pair of the bent portions of the cap and a pair of sides of the main body, respectively, and both the fittable regions are fitted to each other, which is useful for securing the cap and the main body. To be more specific, the metal cap may also be subjected to bending or stamping; in particular, the metal cap may also be bent so as to cover the sides of the package partially. Moreover, in addition to the securing means by the conductive adhesive, other mechanical securing means may also be used in combination. In general, the plurality of fittable areas is provided on each of the sides of the cap and the main body.
The built-in semiconductor optical element includes a laser diode, a photodiode, and an avalanche photodiode, for example. The semiconductor integrated circuit includes a preamplifier IC.